Radio communication system



June 20, 1950 J. c. oBRlEN ETAL RADIO coMMuNrcmoN SYSTEM Filed June 18, 1947 4 Sheets-Sheet 1 June 20, 1950 J. c. oBRlEN ET A1.

RADIO COMMUNICATION SYSTEM l4 Sheets-Sheet 2 Filed June 18, 1947 June 20, 1950 J. c. oBRlEN ET AL RADIO COMMUNICATION SYSTEM 4 Sheets-Shedl 3 Filed June 18, 1947 NEX Gam June 20, 1950 J. c. oBRlEN ET AL RADIO CDMMUNICATION SYSTEM 4 Sheets-Sheet 4 Filed June 18, 1947 FIIIIL .i Z

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JCO'r'en and RB. Hamer Their (ttorneg UNIT-.Eo- STAT-Es 'naar oer-rc E 2,512,530 Ranio=-=coMMUNICA'rIoNRsYs'rEMe J olm C. OBrien, Rochester, and- Robert B, Hauer,-

Scottsville, N. Y., assignors to (Ipreneral#ltailvvayV Signal Company, Rochester, N. Y. Application' J une-18,1947, SeriallN'oz- "7.5553721 communica-tionv facilitiesfora isingle assigned.

carrierfrequency. Atypical application is'foundin .arailway rad-io communicationV system where two controL-or dispatching,ofces=are located rel atively close together'forfgoverningthe-operation of Imobile unitsfwhich` can be selectively. governed@ byonesoice -orthe other, in'accordance. withl their.- location and/or assignment.

i It .is an object' of thevpresentinvention toy transmitintelligence in twocommunication chan nels-simultaneously, substantially without interference; by respective amplitude'modulation and-= frequency modulation of asingle carrier wave.-

AnotherY object of. the present invention.V is to receive. an. amplitude modulated and frequency-- modulatedcarrer Wave and..` detect intelligenceapplied .to the carrier wave by. respective ampli-fv tude modulation and frequency modulation, .withy substantiallyno interference between theamplitude .modulation and vfrequency `modulation channels.

Another, object of f the. present inventiom.4 in

receivinga` carrier Awave both amplitude modu.

latedA andfrequency modulated, isto reducethe.

frequency modulatedcarrier vto a constantbeat frequency, and thereby allow substantially` .un.

distorted detection without interference ofthe..

amplitude modulation channel of the receiver.

Another. object of `the vpresent invention;.in.. apparatusfor .receivinga carrier. wavev having..

both Lamplitude and `frequency modulation,. .is to modulate a secondv oscillator in accordancewi'th.

a phase corrected outputof the frequency modu-. 1ation...ohannel, andV to heterodyne...such .oscil- 1ator.with anintermediate frequency ofthe frequency modulated carrier, and .thereby obtain an amplitude modulated constant frequency. signal.

Another.. objectA of fthe. present Vinvention .is to provideV a two-channel communication system in which .two control stations.tr ansmit .simulta-z neously and` on the-'same carrier wave'A to -a plubeing :common tol thevtwo' control stations, one control sta-tion effecting frequency modulation ofthe carrier wave,` andthe other control station effecting amplitude modulation of the car-vv rier wave;T and vin which-themobile stations havev their respective Y. receivers f adapted 'to f select the reception of. leither `thefrequency modulationy or:Al amplitude modulation' channels v` which'y are a'p-r plied to the'same carrier wave, andthereby sei-W lect as `to .whether vtheyr` are-to be vresponsive ft'of one. controlfstationA or the' other -in accordance.`

with. the assignment and/ ork locationVv ofy such mobile stations.

Other y objects,` purposes and characteristic fea turesf -ofthe 'present invention* will -be'- in vpartV obvious" from 'ther accompanying drawings, and

in part pointed'out asthedescription progresses.`

In describing. theinvention in detail reference 1 is made-to the accompanying 'drawings'in which corresponding'- referenc'ev-y characters and v legendsl refer to corresponding parts throughoutthe-sev-` eral1 illustrations andfin which:

Fig; 1 is a=-block"-diag-ram`illustrating a trans-f mitterY organization of conventional vvelements for. the. radiationof. aecarrier wave` having botheamplitude: and f. frequency modulation;

Fig 2 is a-bloclgdiagramof a receiver-adapted to receivev thel respective-amplitude -modulationf yand frequency modulation channels of a carrien wave having bothl amplitude and frequency mod"- ula'tion;v

Fig.. 3 illustrates schematically, andlpar-tly by block diagram, the circuit organizationfor.V thev reception ofV the respective channelsoff a carrier v Wave having bothyamplitudev and ffrequencymodulation;

Fig; 4 illustratesameans forqapplying .ya check tone` to the Yrespective amplitude modulation and'` frequency modulation channels` of v a transmitter adaptedto--transmit a carrier wave having-both amplitude and-frequency modulationg.y

Fig.` illustrates aA receiver adapted to Aloe-responsive to veither the.am-plitudemodulation or the. frequency modulation channel rof la carrier- Wave subjectlcontemporaneously to both of'suchv modulations.; and

Fig.- 6 illustratesl diagrammatically. a railway-- radio communicationfsystememploying transmit A ting and receiving yapparatus vaffording both am= f plitude `modulation and frequency modulation-- channels simultaneously on a single oarrienwave.

With reference tothe. accompanying. drawings,

schematic circuit diagrams have been used .for thepurpose7 of illustrating. more4 particularlythe. mode vof operation and principles.'` involved. than.- for 'i1- 3 lustrating the specic structure and arrangement of parts that would be employed in practice; and the symbol (B+) isused to designate the connection to the positive terminal of a suitable direct current power supply (not shown), the negative terminal of which is connected to ground.

With reference to Fig. 1, it is illustrated that a carrier wave of a given fundamental frequency is radiated by the antenna TA from the AM-FM transmitter. The AM-FM transmitter comprises the conventional elements of .an FM transmitter including successively, a suitable oscillator for generating a basic carrier frequency, and a reactance modulator for modulating the carrier output of the oscillator in accordance with the various voice frequencies impressed upon the microphone No. 1. The frequency modulated carrier thus produced is fed through conventional frequency multiplier stages as required to obtain the desired frequency deviation ratio, and to obtain the assigned frequency for radiation from the antenna TA. The output of the last frequency multiplier stage is fed through amplifier stages as required, which may include the class C modulated amplifier stage illustrated in Fig. 1.

The Voice frequencies impressed upon the microphone No'. 2, when amplied as required by a suitable audio frequency amplifier are fed into a conventional amplitude modulator, the output of which in turn is effective in any suitable manner, such as by grid or plate modulation, to modulate the already frequency modulated carrier with amplitude modulations in an amplifier stage of the transmitter such, for example, as in the final class C modulated amplifier stage as shown in Fig. 1. Thus, with reference to Fig. 1, the input to the class C modulated amplifier has been frequency modulated, and subsequent to the amplitude modulation effected in that stage, the output carrier of the class C modulated amplifier, which is radiated fby the antenna TA, is frequency modulated according to microphone No. 1 and amplitude modulated according to microphone No. 2. The frequency modulation of the carrier wave may be of a deviation as required'in practice. A deviation of ve to one has been found satisfactory and effective to reduce` noise interference service.V Y

With reference to Fig. 2, an AM-FM receiver is illustrated as having an antenna RA which it is assumed is effective to receive as separate communication channels the amplitude modulation in communication and frequency modulationapplied to a single Y carrier wave of the character such as would be radiated by the antenna TA of Fig. 1. f

For the reception of the frequency modulatio (FM) channel, the receiver apparatus is not unlike that of a conventional frequency modulation receiver in that it includes (see Fig. 2) successive conventional R. F. amplifier, first oscillator and rst mixer, first I. F. amplier, second I. F. amplifier, first limiter, second limiter, F. M. discriminator, A. F. voltage amplifier, and A. F. power amplifier stages, the output ofthe A. F. power amplifier being applied to the speaker No. 1 associated with the frequency modulation channel. It is to be understood that the block Speaker No. 1 of Fig. 2 also includes a suitable output transformer-according to the requirements of practice.

The successive R. F. amplifier, first oscillator and first mixer, first I. F. amplifier, and second I. FQamplifier stages (see Fig. 2) are common to the reception of both AM and FM channels in that the carrier 'in each of these stages of the receiver has both amplitude and frequency modulation.

The output of the A. F. voltage amplifier of the FM channel of the receiver, in addition to feeding into the A. F. power amplifier, has a multiple circuit which feeds through a phase correction network to a reactance modulator grid control circuit, which modulator, in turn, by its output, varies the frequency of the second oscillator to modulate its frequency with the same modulations that have been applied to the carrier by the FM channel.

The output of the second oscillator (see Fig. 2) is fed to a second mixer in which it is combined with the AM-FM carrier output of the second r `The successive stages for reception of the AM channel, following the output of the second mixer are illustratedas third I. F. amplifier, A. M. detector, A. F. voltage amplifier, A. F. power amplifier, and speaker No. 2. The block designated as Speaker No. 2 is assumed to include a suitable output transformer as required. f

With reference to Fig. 3, the AM-FM receiver is more specifically illustrated, particularly as to the novel circuit organization according to the embodiment of the present invention which has been generally described with reference to Fig. 2. For the purpose of simplification of the disclosure of the present invention, certain portions of the v circuit organization of Fig. 3 have been illustrated by block diagram as such elements of the circuit organization can be provided in a manner well known to those skilled in the art, the block diagram-portions of Fig. 3 being readily identified as corresponding to block portions of Fig. 2 which hasfbeen described.

Thus, with reference to Fig. 3, the speaker No. 1 is responsive to the FM channel of a carrier wave which is received by the receiver organization comprising the successive circuit elements as heretofore describedv with reference to Fig. 2.

The second I. F. amplifier is a conventional intermediate frequency amplifier circuit organization having its input fed through an intermediate frequency transformer I to the control grid Vof the tube TI, and having the intermediate frequency transformer I I Yinthe output circuit of that tube."A The winding I2 of the transformer is included inthe anode cathode ltank-circuit of the tube TI', and the positive battery connection (B+) is applied to the lower terminal of such winding. V (B+) is'applied to the screen grid of the tube through the resistor RI, and proper bias `on the grid of the tube TI is suitably provided through the AVC circuit consisting of RI'I, RIS and RIS and condenser CI I, and by the bias resistor R2 `which is connected in the anode cathode circuit and bypassed for radio frequency currents by the condenser C I.

In addition to the tube having its output circuit coupled through the intermediate frequency transformer II tothe first limiter stage, a connection is made by the wire I3 to the anode of the tubeto provide for coupling the output of the second I. F. amplifier to'ground,l through lthe condenser C2 and the resistor R3. The voltage drop 'htube T3, lby resistance n, powerjampuner.

of n the 'tubes'extends f 1 t u h 'loadfresi'stor-RL and a bisj'resistor R5 'toground -The coupling condeneerjCSfallWs passage vof theauliol frequency signal to thev input 'f-y the LA.; F. 'powerampliiier and Svarlsojto.ground *through the entire resistance of tlie'ftpotentionieteri The adjustable tap -oi? the pote'ntioirieterv R6 allows the selection of a part of'theoutput voltag'eof the'A. Ffyoltage'amplier tubefTStoiibe applied to'thefoontolgridcircuit fl thereactar'lce modulator. Such branch of the output of the A. F. Voltagelamplifiertube T3 is also bo yn'ected' Lthr'ouglr"the Y 'cornieriserA rC4 to the contr l-vgridcircuit of theph'ase' correction netwqrkitube ITA. Y

A 'ef'phase *correction netti/forkj tubeV T4 `isv suitablyfbiased, as-the lVariable bias resistor R'I connected between the 'cathode of thev tube T4 and ground, and bypassed by the "condenser CIZ, to operate, preferably, vas'a class A amplier. y A positive battery potential (B+) isapplied to the screen grid of the tube T4 throughthe resistor R8, and `(B+) is also connected to the anode of the tubev through the load resistor R9. The output of the phase correction network tube T4 is coupled by a suitable coupling condenser C5 to the grid resistor REZ of tube T5, to provide phase correction as required to match the frequency modulations of the second oscillator with the frequency modulations of the output of the second I. F. amplifier tube TI.

To consider the phase correction means more c specifically, the control grid of the phase correction tube T4 is biasedby a circuit including resistors R1 and RI I connected in series. The Variable resistor R'I provides adjustable negative bias for the control grid of the tube T4, and an audio yfrequency signal Voltage is also applied to the control grid by the audio frequency Voltage drop across the resistor RII. There is a Voltage drop across' the resistor RII because yof audio signal current flowing in a circuit extending from the output of the A. F. Voltage amplier tube T3 in- -cluding the upper portion of thepotentiometer R6, resistor RI8, condenser C4, and resistor RII, toground. Inthis circuit -the capacitive reactance `of the condenser C4 is" relatively high as compared to the resistance of the resistor RII. The current that iiows in the circuit including the Acondenser C4 and the resistor RI I is relatively low ascomparedto the current flowing in the circuit extending from the output of the A. F. voltage amplifier tube T3 including the upper portion of potentiometer RE, and the condenser CIB, to ground. -`Thus the shifting of ,theA phase' of the Voltage by the 'condenser C4 has Very little effect upon `the phase of the Voltage output of the A. F. voltage aj'i'nplierv tube T3 Y'which bypasses the pbase correction tube-T4.

' lThe Afr. current through theresistorfias, me

to the'-outputfoff'r4}to"ground; combines vectoraily with the currer'itioutputof the Aj-'Fl Voltage' arpli fier :tube T3 tofl provide a'lag'iligfphaseslift 'to' ia degree as-adjusted by Varyirigthe vgai'no'f T4 by adjusting its bias'control" resistor R1. greater pilas ift is accomplished byallowiiignre'current 'oueput-*of'the tube T4 tolflow throughie- 'sister7 R9.

If a leading phasel shift sis' rquired,'such 'shift can be accomplished-by theftrarispos'itien of the condenser C4 and resistor RII v`=witha shunt' resistance Rin parallel `with C4, lwhich is relatively high `as compared tothe?capacitiye'reactaricev4 of the fcondens'er'f'l,l -toY complete the'D. C. Igrid to 'cathode circuit, Ifa -phaSShiftof greater thanfQOls equird, such 1p-hase shift can be ffaccomplished'by e'n'iploying the phase correctionrietworlc'asdescribedby trar'isp"osing` the connections totheioutput lternntls'of the FM dlscniatoi.

I:According tof'rhas'e shifting? principleslslii'n'ilar to those Which-'have been deseribed''theuse ofa highcapacigtivereactance' condensereC'I-in 'lies Wthyarelatvelylow resistance R-E22a11dcncted across the' tankfc'ircuit Vof tha-Second oscillator, providesaneansfby which a lead'irig'veltageldrop acres, e regler-.oriana at tnecsciuatbrfrequency, is 'applied'v to the `control gridfcircuitof thefreactance Y"modulator tube-fT, suchyoltage lead-ing the tank circuit Voltage ofthe oscillator by".

Theoutput of the reactance modulator tube T5, which is connected tothe `tank circuit of thesecond`foscillator,"'therefore has `a capacitivefeffect upon j such -tarkcircuit -to cause oscillation at 'ia lower frequency. YThus, vby introducing" the phase corrected audio frequency voltagevoutput of the A. Fpi/'oltage amplifier tube TS'in'to they control grid circuit ofthe reactance modulator tube'T5, the secondbsci'lla-tor can'have its frequency 'modulated I'in"accordance'with the modula-tions'of the FM 4channel rof the receiver.

A suitable vrnegative bias for the reactance modulator tube is 4provided as by the resistor RIO in the:anodecathode'-circuit, and the yaudi-o Yfrequencyyoltage across the resistor lR9 is applied tothe control grid of the reactance modulator Vrtub'eT through'thefblocking condenser C5 lfor governing-the Acapacitive effect ofl the reactance modulator tube'accordingto the audio frequency 4modulations of the FM vchannel of the receiver. It will be noted that it has been heretofore pointed out that the :audio frequency Voltage acrossthe 'resistor R8 has been vphase corrected in-thatsuch Voltage is a' combination' of the output ofthe voltage famplier tube T3 *and theoutput of the phase correction networktubelTi.

VAlthough it will be readily apparent tothose could be employed, the second oscillator-stage of the receiver illustrated in Fig. 3 is of the Hartley type, and the anode of the tube of the Hartleyoscillator is connected' to thecontrol grid of the second mixer tube throughrthe coupling condenser C14. It is thus provided that thesecond oscillator stage has its frequency modulated in accordance with a phase corrected audio signal of the FM channel of the VAMfIFM receiver, and the modulated frequency Voltage of such oscillator is applied to the grid G2 of the second mixer tube T2 for beating with the frequency modulated and amplitude modulated output of the second I. F. amplifierY tube Tl A negative bias for the control grids of the second mixer tube T2 is provided by the cathode bias resistor R13 which is bypassed for radio frequencies by the condenser C9. The voltage drop across the resistor R3 las supplied by the output of the second I. F. amplifier tube TI excites the control grid of the second mixer tube T2 with a frequency modulated and amplitude modulated voltage. Both frequency and amplitude modulations are present because there has been no preceding discriminator or limiter stages. The suppressor grids of the second mixer tube T2 are connected to ground through a suitable resistor Rl4.

It is thus provided that the second mixer tube T2 has a frequency and amplitude modulated carrier applied to one control grid Gl and a frequency modulated carrier of a different fundamental frequency applied to another control grid G2 by the second oscillator.

When the phase has been corrected to cause the frequency modulations of the second oscillator tube Tt to match the modulations of the carrier applied to the other control grid Gl of the second mixer tube T2, the output of the second mixer tube T2 is of Ia constant beat frequency, irrespective of frequencymodulations of its input circuits, which is the difference between the two fundamental frequencies applied to the respective control grids GI and G2 of the second mixer tube T2. Such beat frequency of course carries the amplitude modulation of the AM channel of the carrier wave because such amplitude modulations are present in the carrier applied to the control grid Gl of the second mixer tube T2.

The output of the second mixer tube T2 sfed through successive I. F. amplier, AM detector, A. F. voltage amplifier and A. F. power amplifier stages to speaker No. 2. The AM detector tube T1, in addition to providing rectication for detecting purposes, serves as an automatic volume control tube to govern the bias of the second I. F. amplifier tube and the third I. F. amplifier tube, the resistors R15 and Ri being provided in the AVC controlcircuits for filtering purposes.

More specically, the `automatic volume control wire I4 is connected to the upper terminal of the volume control potentiometer RII and extends to the control grid circuit of the third I. F. amplier tube T8 through the resistor RI 5, and to the control grid circuit of the second I. F. amplifier tube Tl through the resistors RIS and RIB in series and to the control grid of T2 through RI and R23.

The output audio frequency signal of the AM detector is fed from the movable contact of the volume control potentiometer Rll through conventional A. F. voltage amplifier and A. F. power amplier stages to speaker No. 2.

It is thus providedthat the AM channel of the received AM-FM carrier wave is isolated from the FM channel of that wave upon effectively cancelling the FM channel by heterodyning means in the 'second mixer tube T2. The successive stages of the receiver subsequent to the second mixer -will be readily identified as corresponding to stages of a conventional AM super-heterodyne receiver.

Having thus considered specifically the organization of respective apparatus for transmission and reception of AM-FM modulated carrier waves, consideration will now be given to a typical application of the principles heretofore set forth with certain modifications being applied to the specific apparatus that has been described in order to adapt the two-channel communication system to the specific typical railway radio communication organization according to Fig. 6.

Ina railroad classification yard it is desirable that a conductor located at the point of cutting cars for classification, as at the crest of a hump in a hump yard, be fable to maintain radio communication with respective mobile units at both ends of the yard. The hump locomotive for pushing thev cars over the hump is one of the mobile units, and other mobile units are stationed in the yard, on the other side of the hump for performing odd classification functions as directed by the conductor.

It is general practice to divide large classification yards into two yard sections, one yard being adapted forV the classification of cars to make up trains for traic in one direction, and the other section of the yard being particularly adapted for the classification of cars in making up trains for the opposite direction of trac.

Each of these portions of the yard has its own complement of personnel and equipment, and thus each yard portion has its own conductor and radio communication system whereby he can communicate with mobile .units at various points in that portion of the yard without interference from the other yard portion. For convenience in describing this embodiment of the present invention, one portion of the yard is designated in Fig. 6 as the West Yard, and the other portion is designated as the East Yard.

The conductor of station A is located at the crest of the hump in the wes-t yard, and from such station he can communicate with the various mobile Iunits of the west yard, including a humip mobile unit No. 1 and a yard mobile unit No. 2.

Similarly the :conductor at station B is at the crest of thehump of the east yard, and from suoli station he can maintain radio communication with the mobile units within the east yard including' a hump mobile unit No. 3 and a yard mobile unit No. 4.

Inasinuch as the conductors stations A and B for the respective west and east yards are not too far apart, transmitter and receiver apparatus can readily be common to both stations, and accoi-dingr to the principles of the present invention, theFli/i; channel is assigned to station A and the AM channel to station B. Thus` a conductor speaking into the microphone FM-MK of station A causes FM modulation of the carrier wave transmitted from the Fl transmitter which is common to both stations A and B.

The conductor at station B, by speaking into the microphone Alvi-MK, provides amplitude modulation for the carrier wave radiated from the Fl transmitter. Inasmu'ch as such transmitter has only one frequency assign thereto, it transmits simultaneously both AM and FM.'

qaencv FJV as. required in. affilier, leb'tifla., 0.1@ asgmnset- 0f Such ,wits-f ing4 thercase yitpisp rticillarly de le to. pro,- vide. a singleloudspe r r r verotfeach ofthemobjiefijnita Tehran peseietiyeiyfreseansive te 'citeer thsl'AlYi @banaal 011: the. FM Ghannel, in a'erdamewth the 'marital LaCie-- etwa" Oiaswitcefor, sel@ tine either channel in accordance with the 'Particular e' t, 0rA resi Lyard iin "Wh, thefmobil unit .j l Working Thus it isllastratediri die Gthat Sacher the mobile ,units a. Fi receiver provided. accrbliea t@ With reference 'to Fig. 5 an receiver is v illustrated in which the reception'I on? there oective AMKa-ndFM channels isprovided in a manner which has been heretofore specically de scribed With reference vto Fig. 3, except that a single power' .arnplier and a single loadspealcer is rcomrnon to`v bothchannels of the receiver in that such apparatus isr selectively connected to ther AM'. channel or FM channel by the actuation ofY the. selector svvitchSW.

A mobile unitrworhing in the east yard is interested. in receiving instructionsfrom the conductor on theAM', channel, .and thus the mobile units Nos. 3 and 4;` (see Fig. 6) of that yard have their receivers set Withthe. switch SW of 5 actuated tov its rlower position to select the vactuation of theA speaker in response tov the AMfchannel.- The other mobile units Nos. l and 2, however, are illustrated in. Fig. 6 as being in the West yard, and therefore the receivers of .suchunitshavet .elect sajxitchesfl positioned. to Sele' .i o thelTTi/i c annel, lthe act;

otait di; the u linking.,of,iure"inclut, block diagram, which is stylus-(thea di "signaler thezEr-Mlchannelpand the outp he lovve'hof th YA. F.. lvoltage terni alfQf the Switch SW, infer ycnannel.'

1ere .may .beelements oi, safety roin' the Qon- H939 Slkng k tone, is` effeccn ,in the ,that `conconductor is Uanlobile unit ,n Y u chechtone inu dicato'rg'he' is to ezip'ect'to rece "message, from the conductor, and, failurefto Vrjeceivgje" such rnesis, readily Videntified as a"failure of the oemalunittibfii@patatas 'Illfldsln4 Sutil. @Ondivtiensthemblar traina 'ril We ,d Hamed-r new; est la tomaia@ @een tot by his r o transmitter and. inform the'conductol of les taillis@ to, ,13s 'Yetlis C'Qeeestcrfs message.

' 10 The use Orf, Check., time tor Semewhatsinelafwrp oses is disclosedin the prio-r; patent'tov. "C1 Chappell, No. 2289x121, dated April'Z'l, 194,2, to which reference` can Y tailed description of a check tone.

It is thus desirable that the ,Fl transmitter of Fig. k6v be provided faccordingrto the "transmitter illustrated, partially by'block diagram.' in"Fig','4. This transmitter Will be readily hidtiiied 'as corresponding to theltrarismitter" ofig. Ivhich has been described, except for 'the' additionjfbf'a 'check tone and'push-to-talk buttons "to" V spective FM and A li/Ijch'an'nls.. n' n It is thus provided that the microphone FM- MK is normally disconnected byfthepusl-totalk4 button FM-,TBJ such buttonbeing pro ydci with anormally closed contact 'Whh'co the output of a suitable tone generator t'omtltie input circuit of ther reactane modulator. `Iit is thus provided that tnor'mally,A vvhenfth'e'button FM-A-TB is not depressed for thel transmission of a voice message, a'suitabletnis'appledto the FM channel oi vthe transmitter'for actuft'i'on of relaysl in the' 'respectiveY Inobi'le'un depression of the buttonFlVi-ft-'TBQ rvvle Na cnductork desiresfto talk, opens" the 'cir"mt"frthe tone generator andcloses c 'irc'itfto connect the niicrophnefcircuittothe rac'tarde modulator' Y In. asimar manner atome generator eQrmaH-Y applies a tone voltage to the input'f amplifier, through the., @mehr clases. Qoitt of the push-metalli but@ eMf''l-Q Whrl a conductor desires to talk overithefi/I channel maar;

ANI-MK microphne Qirqut' firf 'gdirri the input to the A. F. amplifler. 'v

With reference towFig. 5,r the receptionot the tones of the respective Gloanlflrelsv` accomplished as if such tones were Voice messages/,audi frequencies being` preferably u sed for the resphctive tones, and the Same tone frequenciesibeing s signed to both andFh/lv channels. inthe receptionof a tone, the secondary '[5 of the output transformer f6' isl efective' 'tofenergize the relayr CT through aI suitable :filter which is tuned tothe tone frequency and'a suit'- able rectifier I'l. The energizaton'of'tl'i'e r `a`y CT in response to a tone transmitted "on,v the or FM channel, asselected'by the` effective by the closure of itsvfrontr c 'ntactl to energize the tone indicator lamp It Will be readily apparenti/hat vvhen a conductor removes 'a check tone transrnitsv a voice message, therelay CT isdeenrlzedbecause ofv failure to recelve atone; dropping away of such relay in addition to opening the circuit for the checktone larnpCiiat front contactl to cause that lamp tobe'eiitinguished, closes the circuit for the rspealrer yat' back" contact I9 to condition the speaker'for the yreception of the voice message. Ith wil1 be' notedthat the speaker is normallydisconnected .f rio'rril the low-er secondary Winding 20 of the outplrlt transformer by the opening of baci; Contactv 9, 'and thus is not responsive to any noisesyfthat Vvg'ould normally be heard in the speaker when no voice messages are being transmittedlf t It is desirable that in addition to providing for radio communication by which the respective conductors can tall; withr their respective` inobile units, it isl desirable that the respectivewrmbile tref units have transmitters by which they can talk with the conductors, and ask for instructions, or clarification of orders that they have received. It is therefore desirable that the respective mobile units have transmitters adapted to transmit to either conductor for instructions, which can best be accomplished by the use of AM-FM transmitters on a distinctive frequency from an AM-FM transmitter which is used by the conductors. Thus in Fig. 6 each mobile unit is illustrated as having 'a F2 transmitter provided according to the transmitter of Fig. 1. Such transmitter has been described as providing separate microphones for the respective AM andI'M channels, whereby the radiated carrier wave can have i applied thereto AM and FM modulations. If it is not required that the AM and FM channels be transmitted simultaneously from a mobile unit, a single microphone may be used, and selectively connected by suitable switching means to the particular AM or FM channel corresponding to the conductor with which that mobile `unit desires to communicate. If it is desired that a trainman in a mobile unit transmit only to the conductor to whom his receiver is selected as being responsive, the selection of a single microphone control circuit for connection to the respective AM and FM channels can be effected by the same switch SW (see Fig. asis used for selecting to which channel the speaker4 is vto be responsive for the reception of messages on the Yfrequency FI.

It is readily understood by those skilled in the art, in view of the fact that the assignment of different carrier frequencies for radio communication is considerably limited, that the two-channel communication system provided by the invention provides for maximum vcommunication facilities for the respective carrier frequencies available.

Thus, assuming that only two distinctive frequencies can be assigned to a classification yard organization such as has beendescribed, with conventional single channel communication systems, it would be necessary,as is standard practice, to assign one frequency for the conductors transmitter for the east yard, and another frequency for the conductors transmitter for the west yard. Then in order to maintain two-way communication between the respective conductors and their mobile units, problems arise as t0 means by which a conductor may butt in upon the transmission of a mobile unit if it is necessary that he provide instructions for such unit immediately. To solve such problem, considerable apparatus is generally required to provide for intermittent, preferably supersonic, interruptions in transmission as a means for allowing a superior party to vbreak in upon the transmission.

By the communication organization for the classification yard as illustrated in Fig. 5, this problem does not arise, as a better use of the two frequencies vassumed to be assigned to the yard is made to provide the tWo-way transmission, and thus there need be no interruption in the ccnductcrs transmitter, and he at any time can speak to his respective mobile units, irrespective of whether or not one of such units is transmitting at that time.

The idea of being able to maintain the conductors Fl transmitter continuously active is also in accord with the desirability of maintaining the check tones continuously applied when no message is being transmitted. If interruption in 12 the transmission is required for the purpose of allowing the initiation of transmission from a mobile unit, in order to maintain the check tone indicator lamps effective, such interruption is necessarily of a very short interval in order that the extinguishing of the check tone lamps under such conditions is not effective to provide an erroneous indication.

Having specifically described two embodiments of a two-way communication system according to the present invention, it is desired to be understood that this form is selected to facilitate in the disclosure of the invention rather than to limit the number of forms which the invention may assume, and it is to be further understood that various adaptations, alterations and modifications may be applied to the specific form shown to meet the requirements of practice, without in any manner departing from the spirit or scope of the present invention, except as limited by the appending claims.

What we claim is:

In a communication receiver for receiving the amplitude modulation channel of a carrier wave having both frequency modulation and amplitude modulation signals, radio frequency and intermediate frequency amplification stages effective to amplify carriers having both amplitude modulation and frequency modulation signals, an oscillator generating a predetermined fundamental frequency, means for modulating the frequency of said oscillator directly in accordance with said frequency modulation signal, and a mixer stage for beating the output carrier of said Ainterniediate frequency amplifier having both amplitude modulation and frequency modulation signals with the modulated frequency of said 0scillator, whereby the output carrier of said mixer stage is at a constant frequency and is amplitude modulated according to the amplitude modulation signal of said carrier wave.

2. In a radio communication receiver of the character described for receiving the amplitude modulation channel of a carrier Wave having both frequency modulation and amplitude modulation signals, radio frequency and intermediate frequency amplifier stages effective to amplify respective carriers having both said amplitude modulation and frequency modulation signals, limiter and discriminator stages effective to reduce said frequency modulation signal as applied to the carrier output of said intermediate frequency amplifier stage to a first audio frequency signal, an oscillator for generating a predetermined fundamental frequency different from the carrier frequency of said intermediate frequency amplifier stage, a reactance modulator for modulating said oscillator in accordance with the modulations of said first audio frequency signal, a mixer stage for beating the output carrie;` of said intermediate frequency amplifier with the modulated frequency of said oscillator, and amplirler and detector stages for respectively amplifying the amplitude modulated signal output of said mixer stage and reducing that signal to a second audio frequency signal having a signal corresponding to the amplitude modulating signal of said carrier wave.

3. In a communication receiver for receiving the amplitude modulation channel of a carrier wave having both frequency modulation and amplitude modulation signals simultaneously applied thereto, radio frequency and intermediate frequency amplifier stages having their output carriers amplitude and frequency modulated acsaid mixer 'stage from said intermediate Ifrefquency amplier; :whereby the-output of said fimixer fstage lis 1a V-lcarrier 'of'constant frequency amplitude"modulated according to said 'amplitude modulation signal of said carrier Wave.

.4. In a'radio communication receiver GIVfor re- A`ceiving'respective frequency modulation and amiplitudelmodulation signals simultaneously on the t'same carrier Wave, anelectro-responsive device fior-each of said signals, frequency modulation fichannel receiving -means `including radio vfre- :,quency and? intermediate 'frequency amplifier,

lmit'erf'and 'discriminator stages respectively for operating one of -s'aid electro-responsive devices infaccordance -With-'the modulation of said frequency modulation signal of said carrier Wave, v`:

said limiter stage being effective to prevent distortionfby said amplitude modul-ation signal, "and amplitude modulation channel receiving means including saidfradio frequency and said intermediate frequency amplifler stages for operating the other of said electro-responsive devices-in accordance with -the Amodulation of said amplitude :modulation signal, said amplitude modulation :receiving fmeans comprising heterodyning means for lrech-icingv the frequency modulation lwhich is fpresentin the-output of :said intermediate frequency amplifier-stage-to a constant intermediate Afrequency yandthereloy prevent distortion of or interference with the reception of said amplitude modulation signal by said frequency modulation signal.

5. Apparatus for receiving lfrequency modulated and amplitude modulated signals simultaneously on the same carrier Wave comprising two electro-responsive devices, a frequency modulation receiver comprising a limiter for preventing distortionfor interference by said amplitude modulated signal for governing one of said devices, and an amplitude modulated receiverfor actuating the other of said electro-responsive devices comprising means for reducing said frequency modulated carrier to a constant beat frequency for preventing distortion or interference by said frequency modulated signal.

6. In a communication receiver; the method of receiving an amplitude modulation signal impressed on a frequency modulated carrier Wave `comprising heterodyning said carrier wave to an intermediate frequency amplitude modulated and frequency modulated carrier, limiting said intermediate frequency carrier to remove said amplitude modulation signal, converting said frequency modulation signal of said intermediate frequency carrier to an audio frequency signal, correcting the phase of said audio frequency, generating a second carrier vat a distinctive fundamental frequency, modulating said second carrier according to the phase corrected audio frequency signal, and heterodyning said intermediate frequency carrier having amplitude and frequency modulation with said second l carrier -h'avng frequency modulation only tofcancel out'the frequency mod- Julation and obtain' a 'constant intermediate' frequency `having only amplitude modulation accordingto the amplitudemodulation of saidcarrier Wave.

"I, In a communication"receiver the method of vreceivingsubstantially without distortion the am- .plitude modulation channel of a carrier Wave having vboth "amplitude and frequency modulation signals simultaneously 'applied thereto comprising, heterodyning said carrier Wave toan intermediate frequency amplitude modulated and Yfrequenc'ymodulatedcarrier, generating a second carrierfrequency, modulating said second carrier in accordance with the lmodulation of said frequency modulation signal of said carrier wave,

'and beating s'aidfintermediate frequency amplitud'e andV frequency 'modulated carrier With said 'frequency modulated second carrier, whereby a carrier is obtained having the frequency modulation channell removed `and only the amplitude modulation channel remaining.

:8. lIn a communication receiver, ltnevnletho'd of receiving an amplitude modulated 'signal impressed-fon a'frequency modulated carrier Wave substantially Without distortion for interference .duef'tofrequency modulation, comprising heterodynin'g' said carrier tof anintermediate amplitude Amodulated''andfrequency modulated'carmen-beating said 'amplitude modulated 'fand v'frequency lmodulated intermediate frequency carrier With a -carr'er having frequency Amodulation corresponding tothe frequencymodulationv of said carrier wave butlnot havingamplitude-modulation to obtain anamplitude modulated carrier -of l substantiallyconstant frequency, and converting said amplitude modulated carrier of substantiallyconst'ant frequency to an audio frequency signal Yir-Vaecor'dance with intelligence received by said amplitude modulated signal on said `carrier Wave.

i9. Thefcombinat'ion with respective frequency modulation and amplitude modulation transmission channels for applying respective frequency modulation and amplitude modulation signals to a radiated carrier Wave, of a microphone and a check tone generator for each of said channels, and means for selectively connecting said microphone or said tone generator to its respective channel, whereby a check tone is transmitted by each channel of said carrier Wave when the microphone for that channel is rendered ineffective.

10. In a communication receiver for receiving selectively frequency modulation or amplitude modulation signals simultaneously applied to a single carrier Wave, an electro-responsive device, frequency modulation channel receiving means comprising a limiter for preventing distortion and interference lby said amplitude modulation signal, amplitude modulation channel receiving means comprising means for reducing said frequency modulated carrier to a constant beat frequency for preventing distortion and interference by said frequency modulation signal, and means for selectively connecting said electro-responsive device to the outputv of said frequency modulation channel receiving means or said amplitude modulation channel receiving means.

11. In apparatus for receiving frequency modulation and amplitude modulation signals simultaneously on the same carrier wave, the combination with frequency modulation channel receiving means comprising a limiter for preventing distortion and interference by said amplitude modulation signal, amplitude modulation channel receiving means comprising means for reducing` said A'frequency modulation carrier to a constant beat frequency for preventing distortion and interference by said frequency modulation signal, of an indicator responsive only to the reception of a signal of a given frequency, an

electro-responsive device, and means for selectively connecting said indicator and said electroresponsive device to the output of either said frequency modulation channel receiving means or said amplitude modulation channel receiving means.

12. In a communication system affording separate channels of communication for two stations with selected groups of mobile stations, a transmitter common to both stations effective when rendered active to transmit a carrier wave having both amplitude modulation and frequency modulation signals, said transmitter including a microphone at a given one of said stations for governing said amplitude modulation signal and another microphone at the other of said stations for governing said frequency modulation signal, a receiver in each of said mobile units having respective channels for the reception of said frequency modulation and amplitude modulation signals, said receiver having an electro-responsive device common to both channels, and manually operable means in each of the mobile units for selectively associating said electro-responsive device With either said frequency modulation or said amplitude modulation channels of said receiver in accordance with Whether that mobile unit is operating in a group to receive its orders from said given station or said other station.

13. In a communication system aording separate channels for communication for two stations with selected groups of mobile units, a transmitter and a receiver common to both stations, said transmitter being tuned to transmit at a rst frequency and said receiver being tuned to be responsive to a second frequency, said transmitter being effective to transmit a carrier YWave having amplitude modulation Yin accordance with' intelligence transmitted from: one of said stationsand frequency modulation in accordance with vintelligence transmitted from the other` of said stations, said receiver being effective to render a loudspeaer in one of said stations responsive only to frequency modulation'of a received carrier wave .at said second frequency having both frequency modulation and amplitude modulation, and saidreceiver being eifective to render a loudspeaker in theother of said stations responsive only to amplitude modulation of a received carrier wave at said second frequency havingv both frequency modulation and amplitude modulation, and a mobile transmitter and a mobile receiver in eachof said mobile units, said mobile transmitter being tuned to transmitv said second frequency and said mobile receiver being tuned to said first frequency, said mobile transmitter-,being effective to transmit a carrierrwave `having either or ,both frequency modulation and amplitude modulation, and said-mobile receiver being eectiveto receive selectively either the frequency modulation signalor the -amplitude modulation signalof the frequencyA modulated and amplitude carrier Wave at said first frequency substantially Without distortion or interference.

JOHN C., OBRIEN. i ROBERT B. 'HANER REFERENCES CITED The followingreferences are of record in the illeyof this patent: l

` UNITED STATES PA'rinxrrsv Wilmotte Oct. 9, 

